As a backrest of a chair, a configuration is known which includes a frame-shaped backrest frame and an extension material stretched across the backrest frame so as to close an opening portion of the backrest frame. The extension material has a mesh-like sheet body and an edge member disposed in an edge portion of the sheet body. The extension material is attached to the backrest frame via the edge member. In this manner, the extension material is stretched across the backrest frame in a state where desired tension is applied to the sheet body.
Incidentally, in the extension material described above, the sheet body receives a load in a planarly perpendicular direction (thickness direction) due to restoring force of the seat body itself or a seated person's seating posture. Therefore, in order to improve reliability and durability of the chair, it is necessary to ensure fixing strength between the sheet body and the edge member and fixing strength between the edge member and the backrest frame.
For example, prior art publications disclose a fixing structure between the sheet body and the edge member and a fixing structure between the edge member and the backrest frame.
For example, the extension material disclosed in Japanese Unexamined Utility Model Application, First Publication No. H6-77699 ('699 Pub) has a covering sheet and a ring sheet disposed on one surface of the covering sheet. The extension material is fixed to a frame by means of welding in a state where the covering sheet is pinched between a flange portion protruding inward from the backrest frame and the ring sheet.
However, according to a configuration disclosed in '699 Pub, the extension material is fixed to the frame in the planarly perpendicular direction of the covering sheet. Consequently, it is difficult to sufficiently support the load acting in the planarly perpendicular direction acting from, for example, one surface side of the covering sheet. In addition, the extension material is directly welded to the frame. Accordingly, it is difficult to align the frame and the extension material with each other, and there is a problem of poor manufacturing efficiency.
The extension material disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-239922 ('922 Pub.) includes a skin member and a resin cord attached to the skin member by being sewed from one surface side of the skin member. The extension material is fixed to a back plate by fitting the resin cord into a groove formed in the back plate.
However, according to a configuration disclosed in '922 Pub., the resin cord is fixed from only one surface side of the skin member. Accordingly, there is still room for improvement, particularly in terms of ensuring the fixing strength between the sheet body and the resin cord.
Furthermore, Japanese Patent No. 4652767 discloses a configuration including the sheet body and the edge member fixed to an edge portion of the sheet body by means of molding.
However, according to a configuration disclosed in Japanese Patent No. 4652767 described above, the edge member is molded to the edge portion of the sheet body. Accordingly, a molding die becomes complicated, which may lead to increased manufacturing cost and poor manufacturing efficiency. In addition, there is a problem in that the edge member is less freely designed.